Answer: Magdeburg hemispheres – Designed by German scientist and mayor of Magdeburg, Otto von Guericke in 1650 to demonstrate the air pump which he had invented, and the concept of atmospheric pressure. [/wikiflickia]

This is the experiment to measure the strength of the vacuum between the two hemispheres. Performed by Otto Von Guericke. Atmospheric pressure is so intense the horses could not pull the two hemispheres apart.